Aircraft manufacturers and commercial airline transportation companies expend considerable effort to ensure that passengers can count on routine and uneventful flights. Nevertheless, unforeseen events can occur on rare occasions. For example, it is impossible to ensure with complete certainty that an unexpected medical emergency requiring the services of a doctor on the ground will not occur at a time at which landing the aircraft is impractical.
In addition, despite careful screening, it is impossible to predict human behavior with complete certainty. Although real-time communication may be available with ground stations to provide advice to the crew or to allow an airport to handle most situations, overt communication while in flight may not be possible or even advisable during certain exceptional occurrences.